As is known, in the last years one of the key elements in telecommunication networks has been the Wavelength Division Multiplexing (WDM) or the Dense Wavelength Division Multiplexing (DWDM) technology, that allows multiplexing wavelengths for transmission over the same optical fiber, thus increasing the density of transmission channels in a transmission window and thus the overall information carried on a fiber. WDM or DWDM devices are getting an increasing interest due to grow of bandwidth demand in telecommunication networks. They are currently deployed in point-to-point configurations allowing two pieces of equipment to utilize fiber resources in a better way. A more flexible configuration is possible by exploiting wavelength routers, which are devices able to switch wavelength channels without terminating them and that are interconnected by fiber links each carrying a number of wavelength channels. Wavelength routers perform optical/electrical/optical (OEO) conversion on an input signal because, usually, the switching operation is performed using an electrical switching fabric. Such configuration is expensive because it requires to terminate all wavelengths at each intermediate node even if the final destination is elsewhere and then to rebuild the signal towards another intermediate node up to the destination. Cheaper solutions could be obtained eliminating unnecessary OEO conversions in consideration of the fact that technology enhancements are extending the range of an optical signal from tens to hundreds (or thousands) kilometers. New network solutions can be based on wavelength routers with OEO conversion, hereinafter referred to as opaque or regenerating nodes, and/or on wavelength routers without OEO conversion, hereinafter referred to as transparent nodes, and/or on mixed wavelength routers, hereinafter referred to as hybrid nodes, that are only partially equipped with OEO converters.
In a network scenario mainly based on transparent nodes, it is essential to evaluate the feasibility of new paths dynamically established across the network. In such a scenario, there is a maximum length that an optical signal can cover without being regenerated, and this maximum length depends on a lot of factors, such as fiber lengths, fiber types, switching elements features, signal bit-rate, and number of wavelengths. In fact, optical signal degrades along its path due to a lot of physical phenomena such as attenuation, dispersion, non linear effects, etc. that can be called signal impairments. When signal impairments in the optical layer prevent an optical path from being setup a regenerating node is required to flush them. Anyway, carriers prefer to limit the deployment of such equipments in the network, hereinafter referred to. as valuable resources, because expensive.
In traditional regenerating networks, i.e. networks based on regenerating nodes, each node regenerates the optical signal, thereby flushing all signal impairments and thus making it possible to establish an arbitrary path across the network. In such networks, routing methods disregards signal impairments because network design guarantees the feasibility of any path. The main focus in such networks is to find wavelength continuity that is a wavelength that is available from the source to the destination along a certain path throughout the network. If nodes are equipped with wavelength converters, i.e., devices designed to convert a wavelength into another one, this task is easier because it is enough to identify a set of wavelengths that can cover the entire path from the source to the destination.
To reduce costs it is possible to build a transparent optical network that makes use of transparent nodes. However, a real network extending over a sufficiently wide area (e.g., a national environment) can be hardly made completely transparent because of the impairments on optical transmission.
In practice, the concept of a translucent or hybrid optical network applies, as shown in FIG. 1, which describes a translucent network 300 that can be partitioned into a certain number of transparent sub-networks 400, which are usually referred to as islands (or domains) of transparency. Each island of transparency has an appropriate extension (with respect of the fibers, equipment, etc.) so that inside each island of transparency any lightpath is guaranteed to be feasible. Along the boundary of each island of transparency some regenerating nodes assure the OEO conversion to restore the characteristics of the original signal. This solution in some way avoids the problem of determining which paths are feasible and which ones are not, simply introducing a network architecture that allows only feasible paths. On the other hand, this approach is sub-optimal respect to the resource utilization of the entire network because clients in two different islands of transparency may be close enough to be connected transparently, i.e., the paths between them may be feasible without the use of expensive OEO conversion. Moreover, such an approach is quite static because introducing changes in each sub-network is not easy.
A more dynamical portrait of the situation depicted in FIG. 1 can be obtained looking at the whole optical network as a combination of transparent and regenerating nodes. These ones are deployed in the network following a suitable strategy based on a statistical analysis of traffic, on geographical and demographical data, etc. This picture seems general enough to represent any optical transparent network feasible in practice, allowing services to be supplied at end users in a cheap way. In such networks it is required not only to find an appropriate wavelength that satisfies continuity constrains but also a path that satisfies signal impairment constraints.
The method used to find a path and a wavelength is usually known as Routing and Wavelength Assignment (RWA) problem. There are a lot of research and studies that try to find out a way to solve this problem in a concurrent way or to split it into two sub-problems: the routing problem and the wavelength assignment problem.
Li Bo, C. Xiaowen, K. Sohraby, Routing and wavelength assignment vs. wavelength converter placement in all-optical networks, IEEE Optical Communications, pp. S22-S28, August 2003, propose a solution for the RWA problem based on the research of a set of routes between each source-destination pair which is usually the k-shortest link-disjoint paths. If a lightpath connection request arrives at a node it should make a decision to choose a path from the pre-computed set of paths and then assign a free wavelength according to the first-fit method to the selected path. The weight associated to the paths depends not only on the wavelength availability but also on the path length.
K. Taira, Y. Zhang, H. Takagi, S. K. Das, Efficient Lightpath Routing in Wavelength-Routed Optical Networks, ICOIN 2002, LNCS 2343, pp. 291-304, 2002, propose a heuristic algorithm that solves the RWA problem. It first solves the routing sub-problem and then the wavelength assignment sub-problem. Both sub-problems are formulated as routing problems and solved using the shortest path routing technique on auxiliary graphs coming from a transformation of the graph associated to the network.
U.S. Pat. No. 6,538,777 discloses a method for allocating channels and paths to connections along candidate channel-paths in a network, where a candidate channel-path comprises a candidate path and candidate channel along the candidate path, and is performed by determining individual effects, on the network, of selecting candidate channel-paths. These include effects on at least one channel-path, other than a candidate channel-path, which shares links with the candidate path. Candidate channel-paths are selected based on the determined effects and allocated. Determination of the effects on the network is based on path capacity. This solution can be used where a single connection has been requested, or alternatively, where multiple connections have been requested. Candidate channel-paths are selected by first calculating a sum of path capacity-dependent values of a set of affected paths in the network for each of plural network states resulting from candidate channel-path allocations, and then selecting the candidate channel-paths yielding a maximum sum.
I. Tomkos, D. Vogiatzis, C. Mas, I. Zacharopoulos, A. Tzanakaki, E. Varvarigos, Performance engineering of metropolitan area optical networks through impairment constraint routing, IEEE Optical Communications, pp. S40-S47, August 2004, demonstrate the use of impairment constraint routing for performance engineering of transparent metropolitan area optical network. In particular, this paper shows the relationship between blocking probability and different network characteristics such as span length, amplifier noise figure, and bit rate, and provides information on the system specification required to achieve acceptable network performance.
A solution that takes into account signal impairments is proposed in US2003/0016414, where the path selection and wavelength assignment to a selected path are performed by mapping the wavelength reach to the demand distribution (agile reach) resulting in a increase in the network reach. The network reach is further increased when on-line measured performance data are used for path selection and wavelength assignment. The connections may be engineered/upgraded individually, by optimizing the parameters of the entire path or of a regenerating stretch of the respective path. The upgrades include changing the wavelength, adjusting the parameters of the regenerating stretch, controlling the launched powers, mapping a certain transmitter and/or receiver to the respective wavelength, selecting the wavelengths on a certain link so as to reduce cross-talk, increasing wavelength spacing, etc.